how to get music copyrighted

Assuming we're talking about the US, your music is copyrighted as soon as it is fixed in some form of expression. Your composition is copyrighted when it is written, and your performance is copyrighted when it is recorded. You do not need to do anything for your works to be copyrighted.

In addition to that default protection, you can register your copyright with the Copyright Office, which gives you additional options. There is a fee and paperwork involved. You can take a look at http://www.copyright.gov/ for the US Copyright Office to find out more about that process.